The Pituitary Flashcards
The pituitary gland is divided into two separate glands:
the anterior and posterior pituitary
The pituitary is located on the
ventral surface of the brain, housed in a protective bony pocket at the base of the skull known as the sella turcica.
Both lobes are in communication with the hypothalamus of the brain
via the median eminence.
The hypothalamic/pituitary/ovarian axis controls
the menstrual cycle.
The pituitary is also known as
the hypophysis due to its location.
The posterior pituitary is composed exclusively of
nervous tissue and is also referred to as the neurohypophysis.
It is the source of two hormones:
oxytocin and vasopressin (also called antidiuretic hormone) ADH).
Vasopressin and oxytocin are
nanopeptides that share a similar amino acid sequence and have a similar biological effect: stimulation of specific smooth muscles.
However,only oxytocin plays a role in female reproduction causing
contraction of the myometrium and the myoepithelial cells surrounding the alveoli of the mammary gland.
The cell bodies of the neurosecretory neurons of the neurohypophysis are located in the
supraoptic and paraventricular nuclei of the anterior hypothalamus.
The axons from these neurons course through the median eminence of the hypothalamus and emerge to form the
pituitary stalk and then terminate within the posterior pituitary
The anterior pituitary includes an
anterior and an intermediate lobe, however the intermediate lobe is rudimentary in humans and will not be reviewed.
Histologically, the anterior pituitary has the cellular appearance of a
protein-producing cell and is termed the adenohypophysis because it is composed of endocrine cells.
Prior to the appearance of menarche (first menses) there is an increase in
andenohypophyseal activity.
The hormones of the anterior pituitary are protein hormones that modulate
other endocrine glands of the body and as such are termed trophic hormones. The name of these trophic hormones are descriptive of their endocrine activity.
The glycoprotein family members share a common alpha subunit comprised of
92 amino acids in the human (96 in almost all other vertebrates).
Their specificity is determined by the unique amino acid sequence
in the beta subunit.
The anterior lobe is the source of six endocrine hormones, three of which play a prominent role in female reproduction:
FSH
LH
PRL
Follicle-stimulating hormone (FSH): Stimulates
the growth and development of ovarian follicles and estrogen secretion.
Luteinizing hormone (LH): elicits
luteinization of the follicle ovulation progesterone secretion by the CL.
Prolactin (Prl; Latin = for lactation’): Essential for
female reproduction, enhances milk production.
Despite its structural similarity to FSH and LH, thyroid-stimulating hormone (TSH) does not
have a documented role in the reproductive system.
However, TSH receptors have been found in
human ovarian tissue suggesting it may participate in the regulation of ovarian function.
An observed impact of raised TSH levels on fecundity in women with a diagnosis of hypothyroidism may be an illustration that the activity of this hormone if not having a regulatory role per se needs to be
maintained in a defined range to prevent adverse actions on Ovarian and endometrial functions.
Human chorionic gonadotropin (hCG): The “pregnancy hormone” is a glycoprotein produced by
the trophectoderm cells of the conceptus that shares structural (92 amino acid alpha chain and 145 amino acid beta chain) and functional overlap with LH and binds to the luteinizing hormone receptor (LHR).
The blood supply to the anterior pituitary is not from a direct arterial connection. The blood vessels entering the anterior pituitary are connected
to a capillary plexus located in the median eminence of the hypothalamus.
The blood vessels that lead from this plexus to the anterior pituitary are a blood portal system:
the hypothalamo-hypophyseal portal blood vessels that empty directly into the vascular sinusoids of the anterior pituitary.
This specialization of the circulation allows the neuroendocrine products of the hypothalamus to be delivered directly to
the anterior pituitary without being diluted in the systemic circulation.
